The present invention relates to a method for applying a light shielding paint to a peripheral area of a lens, particularly a plastic lens, such as used in an optical system of binoculars, cameras, video projectors or television receiver sets, to form a light-shield coat thereon.
Hitherto, two types of light shielding paint for application to lenses have been available: One is a light shielding paint of a type containing an volatile solvent and the other is a two-component paint capable of hardening at room temperature. The application of any one of these light shielding paints requires manual intervention and skill.
More specifically, not only does it often occur that particles of the light shielding paint tend to stain an effective aperture area of a lens, but also the application of the paint is carried out in two separate steps to prevent the effective aperture area of the lens from being touched. (The term "effective aperture area" used in the specification and claims is to be understood as meaning a central area of the lens practically utilized for the passage of rays of light of interest therethrough in a direction parallel to the optical axis of such lens, in contrast to the peripheral area which is utilized for the support thereof on a lens mount or barrel.) Moreover, once the paint has been applied, the drying takes overnight, or several hours at least, constituting one of the major causes of the increased manufacturing cost of lenses coated on the peripheral area with the light shielding paint.
The above discussed problem may be negligible if the production of such lenses remains limited. However, when it comes to a projection type television receiver set, the production of lenses having a relatively large size is desired and, therefore, improvement of the productivity has come about by the use of an injection molding technique, the minimization of the weight and the minimization of the manufacturing cost. Thus, with the increased production of the lenses, a demand has arisen for a light shielding paint capable of hardening quickly.
Since the conventional method for forming a light-shield coat on the peripheral area of a lens is performed by a skilled worker using a brush and a black-colored paint, not only does the thickness of the resultant coat vary from one lens to another, but also a single lens tends to have a varying thickness of the coat. This is a problem in that the coated lens as a whole fails to meet the design requirements to such an extent that, when it is fitted in a lens mount, barrel or any other lens support, it tends to lack stability. Moreover, with the conventional method, the application of the black-colored paint has to be performed so as to form a light-shield coat to a thickness selected in consideration of the possible damage the coat may be subjected to when the coated lens is mounted on the lens mount, barrel or any other lens support. The conventional method has a problem as regards the minimization of the lens manufacturing cost and the uniformity in quality of the lenses.
Furthermore, on the one hand, while the production is increasing, the number of skilled workers competent to apply the paint to the peripheral area of each lens is limited, and on the other hand, no method for the formation of a coat of uniform thickness has yet been established and, therefore, the production tact time cannot be improved at all.
When it comes to conventional coating apparatuses, some employ a spray system and some employ a transfer system using rolls. In the spray system, the paint used is of a type containing a volatile solvent and it has numerous disadvantages in that it is difficult to keep the viscosity of the paint at a predetermined value throughout the coating operation because of the hardening of the material thereof, clogging of the paint tends to readily occur in a spray nozzle, spray of the paint often stains the apparatus, and solvent gases are susceptible to explosion unless the utmost care is taken in the environment in which the coating operation is performed. As regards the transfer system, not only is it difficult to apply the paint simultaneously to two faces of the angled peripheral area of the lens, but also the transfer roll or rolls are susceptible to wear so as to reduce their life time, particularly when the peripheral area of each lens has a plurality of projections such as gates of a few millimeters in length.
When it comes to a screen printing technique, it cannot be applied, without considerable difficulty, in coating a curved surface having indentations and, moreover, some limitations are imposed on both the hardening and drying time of the paint attributable to the reduced production tact time. Apart from the above, so far as the paint of a type capable of hardening when exposed to ultraviolet rays, i.e., a photo-setting paint, is concerned, it can result in a film or coat of a thickness not greater than 30 micrometers and must be repeatedly applied at least two times to form a double-layered coat in order for the lens to have a light-shielding property. However, it often happens that the first and second layers forming the double-layered coat separate from each other.